Hydrogen is able to permeate and seep through almost all metals to a measurable extent. The phenomenon is well known in respect of various steels. In the case of ferritic steels, which include by way of example steel commonly known as carbon steel or mild steel, at temperatures below about 100° C., hydrogen which has permeated into the steel is the cause of a very deleterious failure mechanism known as hydrogen induced cracking (HIC). Usually HIC results from high concentrations of hydrogen entering steel at the internal steel walls of pipes and vessels, assisted by the chemical interaction of specific corrodants known as hydrogen promoters which are contained therein. The most important of these is hydrogen sulphide, a common constituent of crude oil, natural gas, and of refinery processes concerned with the separation of sulphur compounds from oil.
At temperatures exceeding about 100° C. the steel, HIC disappears as a concern. Increase in temperature is attended with a very much increased hydrogen permeability of hydrogen through the steel. Therefore at higher temperatures any type of corrosion which generates hydrogen causes an appreciable amount of hydrogen to permeate steel, such as may be caused by corrosion of steel by organic acids generically known as naphthenic acid, during the distillation of oil containing them, which typically occurs in the temperature range of 200 to 400° C.
In both the above examples of hydrogen permeation it is advantageous to be able to measure the hydrogen flux exiting the external face of a pipe or vessel whose internal face is prospectively corroding. On occasion this may provide an indication of crack risk arising from the corrosion activity, or a measure of the corrosion itself. In either case, the measure of a high hydrogen flux provides a means of assessing the effectiveness of a variety of means of corrosion mitigation to be assessed.
Another application for the flux measurement is upon steel vessels and pipes which are heat treated prior to welding, in order to remove hydrogen in a process known as a hydrogen bakeout. The aim of this process is to avoid hydrogen being trapped in steel in appreciable quantities, which in the absence of such a bakeout may cause subsequent to welding a form of HIC known as stress oriented hydrogen induced cracking (SOHIC). The measurement of flux exiting steel during a hydrogen bakeout may be used to indicate whether a bakeout is needed, if a bakeout is complete, or, most significantly, that further bakeout time is required for the hydrogen exiting the steel to a level associated with a low residual concentration in the steel, whereupon it is safe for welding to proceed without appreciable risk of SOHIC.
In the Applicants' GB Patent 2312279, there is described a hydrogen detection system which is currently in general use in the measurement of hydrogen flux. In this system, air is drawn, by means of a pump, into a probe member that is held against a steel surface. Hydrogen emanating from steel surface under the collector is entrained in the air stream. From there, the air stream is drawn into a central capillary emanating from the probe, and caused to flow across a hydrogen sensor.
In GB 2358060, there is a hydrogen collector comprising a plate which is flexible and has on one major face raised grooves, which upon being appressed to a steel surface, forms the side walls of channels between the steel surface and one collector plate major face. This causes an air stream drawn into the gaseous space between the hydrogen collector major face and the steel surface to efficiently and reliably sweep up hydrogen emanating from the steel surface and into a capillary exiting from the opposing major face of the collector. For most effective collection of hydrogen flux in an air stream conveyed between the deformable plate and tested steel, it is beneficial for the capillary in the collector plate to be placed approximately centrally in an approximately circular plate.
GB 2358060 teaches that the collector plate may be held magnetically against the steel surface. The present Applicants have considered a number of ways of engaging the collector plate by means of magnets. Options for material deployment and arrangements with magnets affixed to springs or spring like materials are limited by the temperature ranges of application, and the consideration that upon deformation to a highly curved steel surface, the forces exerted on springs can oppose the magnetic attraction of the magnets to the steel which hold the collector plate in place. Problems have also been experienced with demagnetisation at high temperature, damage to the hydrogen collector plate caused by magnetic ‘snatching’ against the steel surface and by magnetic withdrawal from the steel surface.